1. Field of the Invention
The present invention relates to an apparatus for purifying exhaust emission from an engine (to be referred to as an exhaust emission purifying apparatus), and in particular, to a technology for reductively purifying nitrogen oxides (NOx) in the exhaust emission using a reducing agent.
2. Description of the Related Art
As a catalytic purification system for removing NOx contained in the exhaust emission of an engine, there has been typically proposed, in Japanese Laid-open (Kokai) Patent Application Publication No. 2005-147118, an exhaust emission purifying apparatus in which a reducing agent or precursor thereof is dosed according to engine operating conditions to the exhaust emission streaming upstream of a reduction catalytic converter disposed in an engine exhaust pipe, so that NOx in the exhaust emission and the reducing agent are subjected to the catalytic-reduction reaction, to thereby purify NOx into harmless components. In this conventional exhaust emission purifying apparatus, in order to promote the use of a normal reducing agent, i.e., a precisely operative reducing agent, a configuration is adopted such that if use of any different kind of aqueous solution or the deficiency of the reducing agent is detected during engine operation and thereafter, once the engine operation is stopped by using an ignition switch, the restart of the engine operation is inhibited any more.
Nevertheless, if the restart of the engine operation is inhibited due to the reducing agent deficiency, for example when the reducing agent becomes deficient immediately before the engine-operated vehicle reaches its destination, it is required for the vehicle to unload cargo and to continue running by keeping the engine operation to a location where the reducing agent may be replenished. In a case where the location at which the reducing agent can be replenished is not so near, an excessive burden is imposed on a vehicle driver and also a fuel is unnecessarily consumed due to the vehicle running irrelevant to the physical distribution. Further, if the vehicle driver did wish to take a rest in a state where the reducing agent is deficient, the drive must refrain stopping of the engine operation. Still further, such a situation is not preferable in view of the global environment.